1. Field of the Invention
This invention relates to a length measuring apparatus comprising measuring means for detecting the position of a marker along a measuring path, a hermetically sealed housing, in which said measuring means and said marker are encapsulated, follow-up means for establishing a non-contacting link between the marker and a detectable point, which is disposed outside the housing and is adjustable relative to the housing along the measuring path and for moving said marker to a position of registration relative to said detectable point.
2. Description of the Prior Art
Such a length measuring apparatus is known from DE-A-38 24 751. The basic advantage of such measuring apparatus resides in that the delicate parts of the measuring apparatus are perfectly protected in the housing in which they are accommodated so that damage or disturbances by dirt, moisture and aggressive fluids will be precluded and, as a result, even highly sensitive measuring means can be used at hazardous locations. The measuring means preferably generate electric measured-value signals, which are delivered by means of lines, which extend through sealed passages out of the housing, to means analyzing or displaying the signals. The follow-up means may consist of one or more transmitter-receiver systems, which are associated with the marker and the detectable point and which generate signals in response to the occurrence of a deviation from a predetermined position of registration. Said signals are shaped by control means and are subsequently used to control the means for driving the marker. Friction wheel drives operated by stepping motors may be employed. It is known to provide optoelectronic or magnetic or inductive or capacitive transmitter-receiver combinations or sensor systems consisting of pairs of associated elements for generating control signals in dependence on the deviation from registration. It will readily be possible to provide only one element of a pair thereof as an element which can be activated and for this purpose is provided with a power supply, illuminating means and receivers, and to provide the other element as a passive receiver or reflector or, in case of a magnetic coupling, to provide one element of the pair of sensors as a permanent magnet, by which a signal that depends on the deviation from registration is generated, e.g., in the other element of the pair. It is desired to effect a follow-up movement with the least possible time lag so that, e.g., the marker will follow with the shortest possible time lag the movement of the detectable point, which is provided at a defined location on a moving member of a machine. In optoelectronic pairs of sensors a suitable part of the housing must be permeable to the radiation which is employed and at least that part of the housing which is disposed between the marker and the detectable point and lies in the range of the displacement of the marker must be permeable as uniformly as possible to the flux used for the detection. The accuracy of the registration and the speed of the response of the follow-up means can be increased by the use of a plurality of pairs of sensors. In the known length measuring apparatus of the kind described first hereinbefore a stationary scale member carrying a measuring scale is provided within the hermetically sealed housing and a scanning device is provided for scanning said scale member and for generating sample or measured-value signals. That scanning unit is displaceable by the follow-up means along the stationary scale member. For this purpose it is possible to provide a drive unit, which comprises a stepping motor or another motor and friction wheels or other means for supporting the unit within the housing. Said friction wheels may be included in a wheeled carriage for displacing the drive unit along the scale member. To ensure a fast start in that the inertia will be overcome quickly and also to ensure the point-for-point alignment of the scanning unit with the detectable point, a controllable piezoelectric element may be included in a coupling provided between the carriage and the scanning unit. In that prior art a relatively high overall expenditure will be required if an exact follow-up control is to be effected and the relatively large mass forces are to be overcome which will inherently be involved in the operation of said known length measuring apparatus.